The Effect of the Physical Form of Biodegradable Polymer Carriers on the Humoral Immune Response to Co-Delivered Antigen

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Abstract

The biomaterial component of a tissue engineered device has been shown to enhance the immune response to a co-delivered model shed antigen. The purpose of this research was to investigate in vivo the differential level of the immune response toward different forms of the biomaterial. A model shed antigen, ovalbumin (OVA), was incorporated into polymeric biomaterial carriers made of 50:50 poly(lactic-co-glycolic acid) (PLGA) in the form of microparticles (MP) or scaffolds (SC). These MP and SC biomaterial carrier vehicles with incorporated antigen were then injected or implanted, respectively, into C57BL6 mice to investigate the differential level of the immune response towards OVA controlled release from PLGA MP and PLGA SC. For each polymeric carrier, the resulting time-dependent systemic humoral immune response towards the incorporated OVA, the OVA-specific IgG concentration and isotypes (IgG2a or IgG1, indicating a predominant Th1 or Th2 response, respectively) were determined using ELISA. To assess the differential level of the immune response depending on the form of PLGA, the total amounts of polymer and OVA delivered were kept constant as well as the release rate of OVA. The in vitro protein release kinetics were studied for both PLGA MPs and PLGA scaffolds to examine the release rate of OVA from the polymeric carriers.
The level of the humoral immune response was higher and sustained for OVA released from PLGA SC which were implanted with associated tissue damage, and lower and transient when the same amount of polymer and OVA were delivered from PLGA MP, which were minimally invasively delivered by injection. This immune response was primarily Th2 helper T cell-dependent as exemplified by the predominance of IgG1 isotype, although for the strong adjuvant, Complete Freunds adjuvant (CFA), and PLGA SC carriers the anti-OVA IgG2a isotype levels were also significant, potentially indicating both a Th2 and Th1 response.
The PLGA SC and PLGA MP exhibited similar protein release kinetics, releasing similar amounts of OVA at each time point. Each carrier incubated contained the same ratio of OVA to polymer. In vitro protein release kinetics experiments suggest that the rate of release of OVA from PLGA SC and PLGA MP was similar, and therefore the enhanced immune response induced by PLGA SC is most likely due to danger signals from implantation which primed the system for an enhanced immune response and not from a difference in concentration of OVA released from the carriers.